Mitochondrial fission, the process by which mitochondria divide, is essential for cell viability but poorly understood at a molecular level. The rate of mitochondrial fission increases during apoptosis and decreases as cells age; inhibition of a human fission protein also inhibits apoptosis. Therefore, a precise knowledge of the mitochondrial fission proteins and the details of their function will provide clear strategies to control apoptosis, which is linked to heart disease, cancer, and AIDS. The long-term goal of this research is to determine the mechanism of mitochondrial fission and its role in apoptosis. Genetic and cytological studies by other investigators have implicated the proteins Fis1, Mdv1, and Dnm1 in mitochondrial fission of budding yeast. The proposed research takes a different approach by determining the biochemical and structural basis for these processes. The guiding hypothesis is that Fis1 recruits Mdv1 and Dnm1 to alter membrane structure as part of their role in fission and apoptosis. As a first step towards testing this hypothesis, a high-resolution structure of Fis1 has recently been determined. The analysis of this structure in context of other studies suggests the following hypotheses: 1) that the Fis1 protein adopts two different conformations, each of which is biologically relevant, 2) that a surface on the Fis1 molecule comprised of evolutionarily conserved amino acids is responsible for binding to itself and other fission proteins, and 3) that regulation of access to this binding surface is an important regulatory mechanism in apoptosis and fission. The proposed research will test these hypotheses by a multi-disciplinary approach involving in vitro assays to determine protein-protein and protein-membrane interactions; structural studies by electron microscopy, NMR spectroscopy, and x-ray crystallography; and in vivo assays for mitochondrial fission and cell death. In validating the proposed hypotheses, the mechanisms and regulation of mitochondrial fission and cellular apoptosis will be identified. Since human homologues of the fission proteins exist, this research will also be a first step in designing new ways to inhibit or induce apoptosis.